In a number of devices, -- for example, in mass spectrometers, -- it is necessary to select particles with particular velocities. Such is the purpose of velocity selectors. Consider an example. Charged particles travel in crossed electric and magnetic fields. An electric field is established between the plates of a parallel-plate capacitor, and a magnetic field establishes between the poles of an electromagnet. The initial vector velocity v of the charged particles is perpendicular to the vectors E and B. An electric force q? (vector) E and the magnetic force FA = qvBsinα, where α is an angle between the vectors v and B, act on a particle that moves between the electric and magnetic fields. Having moved through the capacitor, the particle passes through a small hole in a screen. The condition of the rectilinear motion is given by the formula qE = qvB. The charge and the mass of the particle have no effect on this condition, as it depends only on the speed of the particle. Given the electric and magnetic fields, the selector filters the particles that travel with the following speed: v = E/B. Under certain conditions, the electric and the magnetic forces can be artificially modified to assume equal magnitude, in which case the total force acting on a particle becomes zero. The charged particle will travel inside the capacitor in a straight line and with constant velocity.